In 1979, based on the conception of combining organic chemistry with inorganic chemistry, it is introduced into the field of inorganic chemistry by the inventor that the asymmetric distribution of electron cloud caused by organic aromatic conjugated π-bond charges being attracted and repelled each other generates dipole moment, and thus nonlinear optical effect is generated, And the inventor proposes a concept of finding “similar to the benzene ring” and “boron-oxygen ring” in the field of inorganic chemistry. Microcrystalline powder sample of low-temperature phase barium metaborate (BBO) is occasionally obtained by 68 times experiments through grinding and mixing the main component of α-BBO (boron-oxygen ring), alkali metal and alkaline-earth metal carbonate and sintering the different metal compounds with different ratios. The frequency doubling effects of α-BBO is about 5 times of the one of the KDP. Hereon natrium plays the role of flux. However, it was mistaken for sodium barium metaborate according to the design concept at that time [Reference: Changzhang CHEN, Dongshou GAO and Chuangtian CHEN, Explore of New type Nonlinear Optical Materials—Boron-Oxygen ring Compounds, National Conference on Crystal Growth and Materials, B, 1979, B44:107-111.]. The sample is later identified as β-BBO through the phase diagram and X-ray diffraction analysis [Reference: Shaofang LU, Meiyun HE and Jinling HUANG, Crystal structure of the Low temperature form of barium barate Ba3(B3O6)2, Acta Physica Sinica, 1982, 31(7), 948-955.].
β-BBO is generally prepared by the method of synthetic crystal comprising melting method and czochralski method. In order to obtain larger β-BBO single crystal with an excellent comprehensive property, which is applied to various frequency-converter, the preparation method of BBO has been trying to be improved.
β-BBO prepared by melting method in prior art is as follows:
Growth of β-BBO single crystal using flux seeding method is disclosed in CN85101617.0. The method comprises three steps: mixing materials, seeding crystallon and growing. Wherein Na2O or NaF is employed as an auxiliary flux; the crystallon is placed at the surface of the melted liq.; the orientation of crystallon is in the direction of C-axis.; and the crystallon grows with a speed of 0.03 deg.C. per hour to 0.2 deg.C. per hour. A bowl shaped large single crystal with Φ67 mm and a central depth up to 15 mm and can be steadily grown using this method.
Growth of β-BBO crystal using improved flux seed method is disclosed in CN92112921.1. The method comprises four steps: mixing materials, seeding crystallon, growing and taking out of furnace. Wherein the seed crystal is drilled at its center and a rod is screwed or inserted in the hole to fix the seed. The angle between the orientation of seed crystal and C-axle is from 0 to 60. The cooling rate is from 0.01 to 0.1° C./h and the rotating rate of crystal is from 0 to 30 r/min. Tapping method could comprise directly lifted annealing or converter pouring. High-quality, transparent and large single crystal with Φ80˜150 mm and center thickness of 25˜35 mm can be stably grown using the improving method.
It is crucial in growth process of low-temperature phase barium metaborate (β-BBO) large single crystal using air-cooled crystal method disclosed in CN98104745.9, that a compressed air is continuously blown on the upper surface of the growing crystal through conduct when β-BBO crystal using molten-salt czochralski method or flux seeding method is growing. Continuously cooling the crystal surface results in accelerating the dispersion of crystallization heat produced by crystallization and the impurity-discharging process at crystal growing interface, thus growing rate and quality of crystal is improved. The growth speed of β-BBO crystal rises by 3 to 5 times by using the improved method, namely 1.5 to 2.5 mm/d. UV transmissivity (200-340 mm) of the crystal is raised by 10%, namely up to above 80%.
β-BBO growing method for single crystal using melting method is disclosed in JP1197395. The crystal is prepared by depositing the molten salts of barium chloride and boric acid. Wherein the crystal is crushed, and formed by the rubber-press method; the formed product is primarily sintered at 750-850 deg.C. in an oxygen atmosphere; and the sintered product is crushed, formed, and sintered at 850-1095 deg.C. to obtain the raw rod for producing β-BBO single crystal. The title crystal is grown under the raw rod at <920 deg.C. in a solvent consisting of 25-35 mol % BaO, 40-50 mol % B2O3 and 20-30 mol % Na2O. The growth rate is preferably controlled to 0.1-0.3 mm/h.
Czochralski method in prior arts is as follows:
CN90102894.0 discloses Czochralski method for a β-BBO single crystal with constant fluid level, in which material is intermittently fed by servo-crucible connected with growing-crucible, to achieve feeding and pulling simultaneously. Constant temperature during the growing process results in crystal with good perfection, high transparency, and less inclusions. The yield is significantly increased by the advantage of crystal such as large size etc. The method is again improved in CN94112294.
β-BBO single crystal is widely applied in the fields of laser and nonlinear optics and frequency-converter of ultraviolet and deep-ultraviolet due to its excellent nonlinear optical and thermoelectric properties. However, frequency doubling effect of the coated β-BBO single crystal is still significant attenuated due to its highly deliquescence after a period of using in the humid air; in addition, β-BBO is often broke in processing because its hardness is not enough big. So it is necessary that frequency doubling effect of coated β-BBO single crystal is further improved, and optical damage threshold of β-BBO also is improved.